Trophic factor-induced excitatory synaptogenesis involves postsynaptic modulation of nicotinic acetylcholine receptors

Early Metazoan Origin and Multiple Losses of a Novel Clade of RIM Presynaptic Calcium Channel Scaffolding Protein Homologs

The precise localization of Ca_V2 voltage-gated calcium channels at the synapse active zone requires various interacting proteins, of which, Rab3-interacting molecule or RIM is considered particularly important. In vertebrates, RIM interacts with Ca_V2 channels in vitro via a PDZ domain that binds to the extreme C-termini of the channels at acidic ligand motifs of D/E-D/E/H-WC-_COOH, and knockout of RIM in vertebrates and invertebrates disrupts Ca_V2 channel synaptic localization and synapse function. Here, we describe a previously uncharacterized clade of RIM proteins bearing domain architectures homologous to those of known RIM homologs, but with some notable differences including key amino acids associated with PDZ domain ligand specificity. This novel RIM emerged near the stem lineage of metazoans and underwent extensive losses, but is retained in select animals including the early-diverging placozoan Trichoplax adhaerens, and molluscs. RNA expression and localization studies in Trichoplax and the mollusc snail Lymnaea stagnalis indicate differential regional/tissue type expression, but overlapping expression in single isolated neurons from Lymnaea. Ctenophores, the most early-diverging animals with synapses, are unique among animals with nervous systems in that they lack the canonical RIM, bearing only the newly identified homolog. Through phylogenetic analysis, we find that Ca_V2 channel D/E-D/E/H-WC-_COOH like PDZ ligand motifs were present in the common ancestor of cnidarians and bilaterians, and delineate some deeply conserved C-terminal structures that distinguish Ca_V1 from Ca_V2 channels, and Ca_V1/Ca_V2 from Ca_V3 channels.

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Neurotrophic factors and target-specific retrograde signaling interactions define the specificity of classical and neuropeptide cotransmitter release at identified Lymnaea synapses

Many neurons concurrently and/or differentially release multiple neurotransmitter substances to selectively modulate the activity of distinct postsynaptic targets within a network. However, the molecular mechanisms that produce synaptic heterogeneity by regulating the cotransmitter release characteristics of individual presynaptic terminals remain poorly defined. In particular, we know little about the regulation of neuropeptide corelease, despite the fact that they mediate synaptic transmission, plasticity and neuromodulation. Here, we report that an identified Lymnaea neuron selectively releases its classical small molecule and peptide neurotransmitters, acetylcholine and FMRFamide-derived neuropeptides, to differentially influence the activity of distinct postsynaptic targets that coordinate cardiorespiratory behaviour. Using a combination of electrophysiological, molecular, and pharmacological approaches, we found that neuropeptide cotransmitter release was regulated by cross-talk between extrinsic neurotrophic factor signaling and target-specific retrograde arachidonic acid signaling, which converged on modulation of glycogen synthase kinase 3. In this context, we identified a novel role for the Lymnaea synaptophysin homologue as a specific and synapse-delimited inhibitory regulator of peptide neurotransmitter release. This study is among the first to define the cellular and molecular mechanisms underlying the differential release of cotransmitter substances from individual presynaptic terminals, which allow for context-dependent tuning and plasticity of the synaptic networks underlying patterned motor behaviour.

Dopamine-mediated calcium channel regulation in synaptic suppression in L. stagnalis interneurons

D2 dopamine receptor-mediated suppression of synaptic transmission from interneurons plays a key role in neurobiological functions across species, ranging from respiration to memory formation. In this study, we investigated the mechanisms of D2 receptor-dependent suppression using soma-soma synapse between respiratory interneuron VD4 and LPeD1 in the mollusk Lymnaea stagnalis (L. stagnalis). We studied the effects of dopamine on voltage-dependent Ca²⁺ current and synaptic vesicle release from the VD4. We report that dopamine inhibits voltage-dependent Ca²⁺ current in the VD4 by both voltage-dependent and -independent mechanisms. Dopamine also suppresses synaptic vesicle release downstream of activity-dependent Ca²⁺ influx. Our study demonstrated that dopamine acts through D2 receptors to inhibit interneuron synaptic transmission through both voltage-dependent Ca²⁺ channel-dependent and -independent pathways. Taken together, these findings expand our understanding of dopamine function and fundamental mechanisms that shape the dynamics of neural circuit.

Two proteolytic fragments of menin coordinate the nuclear transcription and postsynaptic clustering of neurotransmitter receptors during synaptogenesis between Lymnaea neurons

Synapse formation and plasticity depend on nuclear transcription and site-specific protein targeting, but the molecular mechanisms that coordinate these steps have not been well defined. The MEN1 tumor suppressor gene, which encodes the protein menin, is known to induce synapse formation and plasticity in the CNS. This synaptogenic function has been conserved across evolution, however the underlying molecular mechanisms remain unidentified. Here, using central neurons from the invertebrate Lymnaea stagnalis, we demonstrate that menin coordinates subunit-specific transcriptional regulation and synaptic clustering of nicotinic acetylcholine receptors (nAChR) during neurotrophic factor (NTF)-dependent excitatory synaptogenesis, via two proteolytic fragments generated by calpain cleavage. Whereas menin is largely regarded as a nuclear protein, our data demonstrate a novel cytoplasmic function at central synapses. Furthermore, this study identifies a novel synaptogenic mechanism in which a single gene product coordinates the nuclear transcription and postsynaptic targeting of neurotransmitter receptors through distinct molecular functions of differentially localized proteolytic fragments.

General anesthetics and cytotoxicity: possible implications for brain health

BACKGROUND

The search for agents that bring about faster induction and quicker recovery in the operating room have yielded numerous anesthetics whose mechanisms of action and potential toxic side effects remain unknown, especially in the young and aging brain.

OBJECTIVE

Taking advantage of our clinical and basic science expertise, here we subject the reader to an interesting perspective vis-à-vis the current applications of general anesthetics, and present evidence for their neurotoxic effects on the developing and elderly brains.

RESULTS

Recent studies have called into question the safety of general anesthetics, especially with regards to potentially significant detrimental impacts on the developing brains of young children, and cognitive decline in the elderly - often following multiple episodes of anesthesia. Despite accumulating evidence from animal studies demonstrating that general anesthesia leads to neurodegeneration and cognitive impairment, to date a clear consensus on the impact of anesthetics in humans remains elusive. Because a direct impact of anesthetics on human neuronal networks is often difficult to deduce experimentally, most laboratories have resorted to animal models - albeit with limited success in translating these findings back to the clinic. Moreover, the precise mechanisms that lead to potential cognitive, learning, and memory decline in young and elderly patients also remain to be fully defined.

CONCLUSIONS

This review will focus primarily on the cytotoxic effects of anesthetics, and offer some practical resolutions that may attenuate their long-term harm. An urgent need for studies on animal models and an increased focus on highly controlled prospective epidemiological studies is also reinforced.

Neonicotinoid insecticides inhibit cholinergic neurotransmission in a molluscan (Lymnaea stagnalis) nervous system

Neonicotinoids are highly potent and selective systemic insecticides, but their widespread use also has a growing impact on non-target animals and contaminates the environment, including surface waters. We tested the neonicotinoid insecticides commercially available in Hungary (acetamiprid, Mospilan; imidacloprid, Kohinor; thiamethoxam, Actara; clothianidin, Apacs; thiacloprid, Calypso) on cholinergic synapses that exist between the VD4 and RPeD1 neurons in the central nervous system of the pond snail Lymnaea stagnalis. In the concentration range used (0.01-1 mg/ml), neither chemical acted as an acetylcholine (ACh) agonist; instead, both displayed antagonist activity, inhibiting the cholinergic excitatory components of the VD4-RPeD1 connection. Thiacloprid (0.01 mg/ml) blocked almost 90% of excitatory postsynaptic potentials (EPSPs), while the less effective thiamethoxam (0.1 mg/ml) reduced the synaptic responses by about 15%. The ACh-evoked membrane responses of the RPeD1 neuron were similarly inhibited by the neonicotinoids, confirming that the same ACh receptor (AChR) target was involved. We conclude that neonicotinoids act on nicotinergic acetylcholine receptors (nAChRs) in the snail CNS. This has been established previously in the insect CNS; however, our data indicate differences in the background mechanism or the nAChR binding site in the snail. Here, we provide the first results concerning neonicotinoid-related toxic effects on the neuronal connections in the molluscan nervous system. Aquatic animals, including molluscs, are at direct risk while facing contaminated surface waters, and snails may provide a suitable model for further studies of the behavioral/neuronal consequences of intoxication by neonicotinoids.

Identification of Drosophila Mutants Affecting Defense to an Entomopathogenic Fungus

Fungi cause the majority of insect disease. However, to date attempts to model host–fungal interactions with Drosophila have focused on opportunistic human pathogens. Here, we performed a screen of 2,613 mutant Drosophila lines to identify host genes affecting susceptibility to the natural insect pathogen Metarhizium anisopliae (Ma549). Overall, 241 (9.22%) mutant lines had altered resistance to Ma549. Life spans ranged from 3.0 to 6.2 days, with females being more susceptible than males in all lines. Speed of kill correlated with within-host growth and onset of sporulation, but total spore production is decoupled from host genotypes. Results showed that mutations affected the ability of Drosophila to restrain rather than tolerate infections and suggested trade-offs between antifungal and antibacterial genes affecting cuticle and gut structural barriers. Approximately, 13% of mutations where in genes previously associated with host pathogen interactions. These encoded fast-acting immune responses including coagulation, phagocytosis, encapsulation and melanization but not the slow-response induction of anti-fungal peptides. The non-immune genes impact a wide variety of biological functions, including behavioral traits. Many have human orthologs already implicated in human disorders; while others were mutations in protein and non-protein coding genes for which disease resistance was the first biological annotation.

A planar microelectrode array for simultaneous detection of electrically evoked dopamine release from distinct locations of a single isolated neuron

Neurotransmission is a key process of communication between neurons. Although much is known about this process and the influence it has on the function of the body, little is understood about the dynamics of signalling from structural regions of a single neuron. In this study we have fabricated and characterised a microelectrode array (MEA) which was utilised for simultaneous multi-site recordings of dopamine release from an isolated single neuron. The MEA consisted of gold electrodes that were created in plane with the insulation layer using a chemical mechanical planarization process. The detection limit for dopamine measurements was 11 ± 3 nM and all the gold electrodes performed in a consistent fashion during amperometric recordings of 100 nM dopamine. Fouling of the gold electrode was investigated, where no significant change in the current was observed over 4 hours when monitoring 100 nM dopamine. The MEA was accessed using freshly isolated dopaminergic somas from the pond snail, Lymnaea stagnalis, where electrically evoked dopamine release was clearly observed. Measurements were conducted at four structural locations of a single isolated neuron, where electrically evoked dopamine release was observed from the cell body, axonal regions and the terminal. Over time, the release of dopamine varied over the structural regions of the neuron. Such information can provide an insight into the signalling mechanism of neurons and how they potentially form synaptic connections.

Nanomedicine for treating spinal cord injury

Spinal cord injury results in significant mortality and morbidity, lifestyle changes, and difficult rehabilitation. Treatment of spinal cord injury is challenging because the spinal cord is both complex to treat acutely and difficult to regenerate. Nanomaterials can be used to provide effective treatments; their unique properties can facilitate drug delivery to the injury site, enact as neuroprotective agents, or provide platforms to stimulate regrowth of damaged tissues. We review recent uses of nanomaterials including nanowires, micelles, nanoparticles, liposomes, and carbon-based nanomaterials for neuroprotection in the acute phase. We also review the design and neural regenerative application of electrospun scaffolds, conduits, and self-assembling peptide scaffolds.

Acetylcholine elongates neuronal growth cone filopodia via activation of nicotinic acetylcholine receptors

In addition to acting as a classical neurotransmitter in synaptic transmission, acetylcholine (ACh) has been shown to play a role in axonal growth and growth cone guidance. What is not well understood is how ACh acts on growth cones to affect growth cone filopodia, structures known to be important for neuronal pathfinding. We addressed this question using an identified neuron (B5) from the buccal ganglion of the pond snail Helisoma trivolvis in cell culture. ACh treatment caused pronounced filopodial elongation within minutes, an effect that required calcium influx and resulted in the elevation of the intracellular calcium concentration ([Ca]i ). Whole-cell patch clamp recordings showed that ACh caused a reduction in input resistance, a depolarization of the membrane potential, and an increase in firing frequency in B5 neurons. These effects were mediated via the activation of nicotinic acetylcholine receptors (nAChRs), as the nAChR agonist dimethylphenylpiperazinium (DMPP) mimicked the effects of ACh on filopodial elongation, [Ca]i elevation, and changes in electrical activity. Moreover, the nAChR antagonist tubucurarine blocked all DMPP-induced effects. Lastly, ACh acted locally at the growth cone, because growth cones that were physically isolated from their parent neuron responded to ACh by filopodial elongation with a similar time course as growth cones that remained connected to their parent neuron. Our data revealed a critical role for ACh as a modulator of growth cone filopodial dynamics. ACh signaling was mediated via nAChRs and resulted in Ca influx, which, in turn, caused filopodial elongation.

Molluscan neurons in culture: shedding light on synapse formation and plasticity

From genes to behaviour, the simple model system approach has played many pivotal roles in deciphering nervous system function in both invertebrates and vertebrates. However, with the advent of sophisticated imaging and recording techniques enabling the direct investigation of single vertebrate neurons, the utility of simple invertebrate organisms as model systems has been put to question. To address this subject meaningfully and comprehensively, we first review the contributions made by invertebrates in the field of neuroscience over the years, paving the way for similar breakthroughs in higher animals. In particular, we focus on molluscan (Lymnaea, Aplysia, and Helisoma) and leech (Hirudo) models and the pivotal roles they have played in elucidating mechanisms of synapse formation and plasticity. While the ultimate goal in neuroscience is to understand the workings of the human brain in both its normal and diseased states, the sheer complexity of most vertebrate models still makes it difficult to define the underlying principles of nervous system function. Investigators have thus turned to invertebrate models, which are unique with respect to their simple nervous systems that are endowed with a finite number of large, individually identifiable neurons of known function. We start off by discussing in vivo and semi-intact preparations, regarding their amenability to simple circuit analysis. Despite the 'simplicity' of invertebrate nervous systems however, it is still difficult to study individual synaptic connections in detail. We therefore emphasize in the next section, the utility of studying identified invertebrate neurons in vitro, to directly examine the development, specificity, and plasticity of synaptic connections in a well-defined environment, at a resolution that it is still unapproachable in the intact brain. We conclude with a discussion of the future of invertebrates in neuroscience in elucidating mechanisms of neurological disease and developing neuron-silicon interfaces.

Network, cellular, and molecular mechanisms underlying long-term memory formation

The neural network stores information through activity-dependent synaptic plasticity that occurs in populations of neurons. Persistent forms of synaptic plasticity may account for long-term memory storage, and the most salient forms are the changes in the structure of synapses. The theory proposes that encoding should use a sparse code and evidence suggests that this can be achieved through offline reactivation or by sparse initial recruitment of the network units. This idea implies that in some cases the neurons that underwent structural synaptic plasticity might be a subpopulation of those originally recruited; However, it is not yet clear whether all the neurons recruited during acquisition are the ones that underwent persistent forms of synaptic plasticity and responsible for memory retrieval. To determine which neural units underlie long-term memory storage, we need to characterize which are the persistent forms of synaptic plasticity occurring in these neural ensembles and the best hints so far are the molecular signals underlying structural modifications of the synapses. Structural synaptic plasticity can be achieved by the activity of various signal transduction pathways, including the NMDA-CaMKII and ACh-MAPK. These pathways converge with the Rho family of GTPases and the consequent ERK 1/2 activation, which regulates multiple cellular functions such as protein translation, protein trafficking, and gene transcription. The most detailed explanation may come from models that allow us to determine the contribution of each piece of this fascinating puzzle that is the neuron and the neural network.

A novel form of presynaptic CaMKII-dependent short-term potentiation between Lymnaea neurons

Short-term plasticity is thought to form the basis for working memory, the cellular mechanisms of which are the least understood in the nervous system. In this study, using in vitro reconstructed synapses between the identified Lymnaea neuron visceral dorsal 4 (VD4) and left pedal dorsal 1 (LPeD1), we demonstrate a novel form of short-term potentiation (STP) which is 'use'- but not time-dependent, unlike most previously defined forms of short-term synaptic plasticity. Using a triple-cell configuration we demonstrate for the first time that a single presynaptic neuron can reliably potentiate both inhibitory and excitatory synapses. We further demonstrate that, unlike previously described forms of STP, the synaptic potentiation between Lymnaea neurons does not involve postsynaptic receptor sensitization or presynaptic residual calcium. Finally, we provide evidence that STP at the VD4-LPeD1 synapse requires presynaptic calcium/calmodulin dependent kinase II (CaMKII). Taken together, our study identifies a novel form of STP which may provide the basis for both short- and long-term potentiation, in the absence of any protein synthesis-dependent steps, and involve CaMKII activity exclusively in the presynaptic cell.

Serotonin modulates transmitter release at central Lymnaea synapses through a G-protein-coupled and cAMP-mediated pathway

Neuromodulation is central to all nervous system function, although the precise mechanisms by which neurotransmitters affect synaptic efficacy between central neurons remain to be fully elucidated. In this study, we examined the neuromodulatory action of serotonin [5-hydroxytryptamine (5-HT)] at central synapses between identified neurons from the pond snail Lymnaea stagnalis. Using whole-cell voltage-clamp and sharp electrode recording, we show that 5-HT strongly depresses synaptic strength between cultured, cholinergic neuron visceral dorsal 4 (VD4 - presynaptic) and its serotonergic target left pedal dorsal 1 (LPeD1 - postsynaptic). This inhibition was accompanied by a reduction in synaptic depression, but had no effect on postsynaptic input resistance, indicating a presynaptic origin. In addition, serotonin inhibited the presynaptic calcium current (I(Ca)) on a similar time course as the change in synaptic transmission. Introduction of a non-condensable GDP analog, GDP-beta-S, through the presynaptic pipette inhibited the serotonin-mediated effect on I(Ca.) Similar results were obtained with a membrane-impermeable inactive cAMP analog, 8OH-cAMP. Furthermore, stimulation of the serotonergic postsynaptic cell also inhibited presynaptic currents, indicating the presence of a negative feedback loop between LPeD1 and VD4. Taken together, this study provides direct evidence for a negative feedback mechanism, whereby the activity of a presynaptic respiratory central pattern-generating neuron is regulated by its postsynaptic target cell. We demonstrate that either serotonin or LPeD1 activity-induced depression of presynaptic transmitter release from VD4 involves voltage-gated calcium channels and is mediated through a G-protein-coupled and cAMP-mediated system.

Peripheral oxygen-sensing cells directly modulate the output of an identified respiratory central pattern generating neuron

Breathing is an essential homeostatic behavior regulated by central neuronal networks, often called central pattern generators (CPGs). Despite ongoing advances in our understanding of the neural control of breathing, the basic mechanisms by which peripheral input modulates the activities of the central respiratory CPG remain elusive. This lack of fundamental knowledge vis-à-vis the role of peripheral influences in the control of the respiratory CPG is due in large part to the complexity of mammalian respiratory control centres. We have therefore developed a simpler invertebrate model to study the basic cellular and synaptic mechanisms by which a peripheral chemosensory input affects the central respiratory CPG. Here we report on the identification and characterization of peripheral chemoreceptor cells (PCRCs) that relay hypoxia-sensitive chemosensory information to the known respiratory CPG neuron right pedal dorsal 1 in the mollusk Lymnaea stagnalis. Selective perfusion of these PCRCs with hypoxic saline triggered bursting activity in these neurons and when isolated in cell culture these cells also demonstrated hypoxic sensitivity that resulted in membrane depolarization and spiking activity. When cocultured with right pedal dorsal 1, the PCRCs developed synapses that exhibited a form of short-term synaptic plasticity in response to hypoxia. Finally, osphradial denervation in intact animals significantly perturbed respiratory activity compared with their sham counterparts. This study provides evidence for direct synaptic connectivity between a peripheral regulatory element and a central respiratory CPG neuron, revealing a potential locus for hypoxia-induced synaptic plasticity underlying breathing behavior.

Ryanodine receptor-transmitter release site coupling increases quantal size in a synapse-specific manner

The mechanisms by which presynaptic neurones differentially regulate synaptic transmission with multiple postsynaptic targets in the brain are not fully understood. Using intracellular sharp electrode and whole-cell voltage-clamp recordings of soma-soma synapses between identified Lymnaea neurones, we provide direct evidence that quantal size is regulated presynaptically through the coupling of multiple release sites. This coupling effectively multiplies quantal size, thereby providing significant influence over parameters of synaptic transmission that are influenced by quantal size, such as the variance in transmitter release at stationary release probabilities. Variation in the degree of coupling is dependent on the identity of the postsynaptic cell, even though the variation in quantal size is of presynaptic origin. We have therefore demonstrated the presence of a novel mechanism by which presynaptic neurones may differentially regulate quantal size at select synaptic connections, in turn providing them with a means of regulating synaptic transmission with multiple postsynaptic cells.

Activation of epidermal growth factor receptor mediates receptor axon sorting and extension in the developing olfactory system of the moth Manduca sexta

During development of the adult olfactory system of the moth Manduca sexta, olfactory receptor neurons extend axons from the olfactory epithelium in the antenna into the brain. As they arrive at the brain, interactions with centrally derived glial cells cause axons to sort and fasciculate with other axons destined to innervate the same glomeruli. Here we report studies indicating that activation of the epidermal growth factor receptor (EGFR) is involved in axon ingrowth and targeting. Blocking the EGFR kinase domain pharmacologically leads to stalling of many axons in the sorting zone and nerve layer as well as abnormal axonal fasciculation in the sorting zone. We also find that neuroglian, an IgCAM known to activate the EGFR through homophilic interactions in other systems, is transiently present on olfactory receptor neuron axons and on glia during the critical stages of the sorting process. The neuroglian is resistant to extraction with Triton X-100 in the sorting zone and nerve layer, possibly indicating its stabilization by homophilic binding in these regions. Our results suggest a mechanism whereby neuroglian molecules on axons and possibly sorting zone glia bind homophilically, leading to activation of EGFRs, with subsequent effects on axon sorting, pathfinding, and extension, and glomerulus development.

Identification and functional expression of a family of nicotinic acetylcholine receptor subunits in the central nervous system of the mollusc Lymnaea stagnalis

We described a family of nicotinic acetylcholine receptor (nAChR) subunits underlying cholinergic transmission in the central nervous system (CNS) of the mollusc Lymnaea stagnalis. By using degenerate PCR cloning, we identified 12 subunits that display a high sequence similarity to nAChR subunits, of which 10 are of the alpha-type, 1 is of the beta-type, and 1 was not classified because of insufficient sequence information. Heterologous expression of identified subunits confirms their capacity to form functional receptors responding to acetylcholine. The alpha-type subunits can be divided into groups that appear to underlie cation-conducting (excitatory) and anion-conducting (inhibitory) channels involved in synaptic cholinergic transmission. The expression of the Lymnaea nAChR subunits, assessed by real time quantitative PCR and in situ hybridization, indicates that it is localized to neurons and widespread in the CNS, with the number and localization of expressing neurons differing considerably between subunit types. At least 10% of the CNS neurons showed detectable nAChR subunit expression. In addition, cholinergic neurons, as indicated by the expression of the vesicular ACh transporter, comprise approximately 10% of the neurons in all ganglia. Together, our data suggested a prominent role for fast cholinergic transmission in the Lymnaea CNS by using a number of neuronal nAChR subtypes comparable with vertebrate species but with a functional complexity that may be much higher.

Synapse formation and plasticity: the roles of local protein synthesis

From simple reflexes in lower animals to complex motor patterns and learning and memory in higher animals, all nervous system functions hinge upon fundamental, albeit specialized, neuronal units termed synapses. The term synapse denotes the structural and functional building block upon which pivots the enormous information-processing capabilities of our brain. It is the neuronal communications through synapses that ultimately determine who we are and how we react and adapt to our ever-changing environment. Synapses are not only the epic center of our intellect, but they also control myriad traits of our personality, ranging from sinfulness to sainthood (see, e.g., Hamer 2004). Simply put-we are what our synapses deem us to be (LeDoux 2003)! Notwithstanding the reasoning that some aspects of the synaptic arrangement may be genetically hardwired, an overwhelming body of knowledge does nevertheless provide ample plausible evidence that synapses are highly plastic entities undergoing rapid adaptive changes throughout life. It is this adaptability that endows our brain with its "uncanny" powers.

Transplantation and restoration of functional synapses between an identified neuron and its targets in the intact brain of Lymnaea stagnalis

Most information available to date regarding the cellular and synaptic mechanisms of target cell selection and specific synapse formation has primarily come from in vitro cell culture studies. Whether fundamental mechanisms of synapse formation revealed through in vitro studies are similar to those occurring in vivo has not yet been determined. Taking advantage of the regenerative capabilities of adult molluscan neurons, we demonstrate that when transplanted into the host ganglia an identified neuron reestablishes its synaptic connections with appropriate targets in vivo. This synaptogenesis, however, was possible only if the targets were denervated from the host cell. Specifically, the giant dopamine neuron right pedal dorsal 1 (RPeD1) located in the pedal ganglia was isolated from a donor brain and transplanted into the visceral ganglia of the recipient brain. We discovered that within 2-4 days the transplanted RPeD1 exhibited extensive regeneration. However, simultaneous intracellular recordings failed to reveal synapses between the transplanted cell and its targets in the visceral ganglia, despite physical overlap between the neurites. To test whether the failure of a transplanted cell to innervate its target was due to the fact that the targets continued to receive input from the native RPeD1, the latter soma was surgically removed prior to the transplantation of RPeD1. Even after the removal of host soma, the transplanted RPeD1 failed to innervate the targets such as visceral dorsal 4 (VD4)-despite extensive regeneration by the transplanted cell. However, when RPeD1 axon was allowed to degenerate completely, the transplanted RPeD1 successfully innervated all of its targets and these synapses were similar to those seen between host RPeD1 and its targets. Taken together, our data demonstrate that the transplanted cells will innervate their potential targets only if the targets were denervated from the host cell. These data also lend support to the idea that, irrespective of their physical location in the brain, the displaced neurons are able to regenerate, recognize their targets, and establish specific synapses in the nervous system.

Acetylcholine binding protein (AChBP): a secreted glial protein that provides a high-resolution model for the extracellular domain of pentameric ligand-gated ion channels

Acetylcholine binding protein (AChBP) has recently been identified from molluskan glial cells. Glial cells secrete it into cholinergic synapses, where it plays a role in modulating synaptic transmission. This novel mechanism resembles glia-dependent modulation of glutamate synapses, with several key differences. AChBP is a homolog of the ligand binding domain of the pentameric ligand-gated ion-channels. The crystal structure of AChBP provides the first high-resolution structure for this family of Cys-loop receptors. Nicotinic acetylcholine receptors and related ion-channels such as GABAA, serotonin 5HT3, and glycine can be interpreted in the light of the 2.7 A AChBP structure. The structural template provides critical details of the binding site and helps create models for toxin binding, mutational effects, and molecular gating.

Synaptogenesis in the CNS: an odyssey from wiring together to firing together

To acquire a better comprehension of nervous system function, it is imperative to understand how synapses are assembled during development and subsequently altered throughout life. Despite recent advances in the fields of neurodevelopment and synaptic plasticity, relatively little is known about the mechanisms that guide synapse formation in the central nervous system (CNS). Although many structural components of the synaptic machinery are pre-assembled prior to the arrival of growth cones at the site of their potential targets, innumerable changes, central to the proper wiring of the brain, must subsequently take place through contact-mediated cell-cell communications. Identification of such signalling molecules and a characterization of various events underlying synaptogenesis are pivotal to our understanding of how a brain cell completes its odyssey from "wiring together to firing together". Here we attempt to provide a comprehensive overview that pertains directly to the cellular and molecular mechanisms of selection, formation and refinement of synapses during the development of the CNS in both vertebrates and invertebrates.

Anesthetic treatment blocks synaptogenesis but not neuronal regeneration of cultured Lymnaea neurons

Trauma and injury necessitate the use of various surgical interventions, yet such procedures themselves are invasive and often interrupt synaptic communications in the nervous system. Because anesthesia is required during surgery, it is important to determine whether long-term exposure of injured nervous tissue to anesthetics is detrimental to regeneration of neuronal processes and synaptic connections. In this study, using identified molluscan neurons, we provide direct evidence that the anesthetic propofol blocks cholinergic synaptic transmission between soma-soma paired Lymnaea neurons in a dose-dependent and reversible manner. These effects do not involve presynaptic secretory machinery, but rather postsynaptic acetylcholine receptors were affected by the anesthetic. Moreover, we discovered that long-term (18-24 h) anesthetic treatment of soma-soma paired neurons blocked synaptogenesis between these cells. However, after several hours of anesthetic washout, synapses developed between the neurons in a manner similar to that seen in vivo. Long-term anesthetic treatment of the identified neurons visceral dorsal 4 (VD4) and left pedal dorsal 1 (LPeD1) and the electrically coupled Pedal A cluster neurons (PeA) did not affect nerve regeneration in cell culture as the neurons continued to exhibit extensive neurite outgrowth. However, these sprouted neurons failed to develop chemical (VD4 and LPeD1) and electrical (PeA) synapses as observed in their control counterparts. After drug washout, appropriate synapses did reform between the cells, although this synaptogenesis required several days. Taken together, this study provides the first direct evidence that the clinically used anesthetic propofol does not affect nerve regeneration. However, the formation of both chemical and electrical synapses is severely compromised in the presence of this drug. This study emphasizes the importance of short-term anesthetic treatment, which may be critical for the restoration of synaptic connections between injured neurons.

Synapse number and synaptic efficacy are regulated by presynaptic cAMP and protein kinase A

The mechanisms by which neurons regulate the number and strength of synapses during development and synaptic plasticity have not yet been defined fully. This lack of fundamental knowledge in the fields of neurodevelopment and synaptic plasticity can be attributed, in part, to compensatory mechanisms by which neurons accommodate for the loss of function in their synaptic partners. This is generally achieved either by scaling up neuronal transmitter release capabilities or by enhancing the postsynaptic responsiveness. Here, we demonstrate that regulation of synaptic strength and number between identified Lymnaea neurons visceral dorsal 4 (VD4, the presynaptic cell) and left pedal dorsal 1 (LPeD1, the postsynaptic cell) requires presynaptic activation of a cAMP-PKA-dependent signal. Experimental activation of the cAMP-PKA pathway resulted in reduced synaptic efficacy, whereas inhibition of the cAMP-PKA cascade permitted hyperinnervation and an overall enhancement of synaptic strength. Because synaptic transmission between VD4 and LPeD1 does not require a cAMP-PKA pathway, our data show that these messengers may play a novel role in regulating the synaptic efficacy during early synaptogenesis and plasticity.

Synapse formation between isolated axons requires presynaptic soma and redistribution of postsynaptic AChRs

The involvement of neuronal protein synthetic machinery and extrinsic trophic factors during synapse formation is poorly understood. Here we determine the roles of these processes by reconstructing synapses between the axons severed from identified Lymnaea neurons in cell culture, either in the presence or absence of trophic factors. We demonstrate that, although synapses are maintained between isolated pre- and postsynaptic axons for several days, the presynaptic, but not the postsynaptic, cell body, however, is required for new synapse formation between soma-axon pairs. The formation of cholinergic synapses between presynaptic soma and postsynaptic axon requires gene transcription and protein synthesis solely in the presynaptic neuron. We show that this synaptogenesis is contingent on extrinsic trophic factors present in brain conditioned medium (CM). The CM-induced excitatory synapse formation is mediated through receptor tyrosine kinases. We further demonstrate that, although the postsynaptic axon does not require new protein synthesis for synapse formation, its contact with the presynaptic cell in CM, but not in defined medium (no trophic factors), differentially alters its responsiveness to exogenously applied acetylcholine at synaptic compared with extrasynaptic sites. Together, these data suggest a synergetic action of cell-cell signaling and trophic factors to bring about specific changes in both pre- and postsynaptic neurons during synapse formation.

Specificity of synapse formation between Lymnaea heart motor neuron and muscle fiber is maintained in vitro in a soma-muscle configuration

Precise neuronal connectivity during development is subservient to all nervous system functions in adult animals. However, the cellular mechanisms that mastermind this neuronal connectivity remain largely unknown. This lack of fundamental knowledge regarding nervous system development is due in part to the immense complexity of mammalian brain, as cell-cell interactions between defined sets of pre- and postsynaptic partners are often difficult to investigate directly. In this study, we developed a novel model system which has allowed us to reconstruct synapses between identified motor neurons and their target heart muscle cell in a soma-muscle configuration. Utilizing this soma-myocardial cell synapse model, we demonstrate that synapses between somata and heart muscle cells can be reconstructed in cell culture. The soma-myocardial cell synapses required 12-24 h to develop and thus differed temporally from conventional neuromuscular synapses (seconds to a few minutes). We also demonstrate that the synapses are target cell-type-specific and are most likely independent of transmitter phenotypic characteristics of presynaptic neurons.